Therein lies a clue! Hands up those who ALWAYS tighten a screw coupling when shunting? Unlike 3 link couplings, a screw coupling can act as a strut, especially if not well lubricated, and can ride up the back of a coupling when being propelled and stay in that position. That is why standard RCH couplings are provided with over-riders. GWR ones, along with quite a few industrials, aren't. Some pre-grouping ones are similar but most have now been replaced with the RCH type.